When drilling or completing wells that traverse earth formations, an upper portion of the primary well bore is drilled from the earth's surface to a selected depth, and then lined with a first section of pipe, commonly referred to as surface casing. The surface casing is then cemented into place in the well bore. The next succeeding section of the primary well bore is drilled to a selected depth below the surface casing and then lined with a string of pipe, commonly referred to as a liner. For each succeeding section of well bore that is drilled, a liner string is installed into the open borehole, below the surface casing or a previously installed liner string. During this process, each liner string may be cemented into place in the well bore.
Once the primary well bore is drilled and lined, a secondary well bore, such as a lateral well bore, for example, may be drilled and also lined with a liner. To perform a liner installation in either a primary or a secondary well bore, a running tool is releasably attached to a liner string. The running tool is connected to a work string or drill pipe that lowers the liner from the earth's surface into the open borehole below the surface casing or a previously installed liner string. The liner string may be rotated via the running tool to clear any obstructions in the borehole and to reduce friction as the liner string is lowered toward the bottom of the borehole.
Each liner string is connected at its upper end to a tubular liner hanger or another type of connection tubular, such as a lateral tube that extends between a primary well bore and a secondary well bore. The liner is lowered on the running tool via the work string until the liner hanger or connection tubular is adjacent to or near the lower end of the surface casing or a previously installed liner string. Then the liner hanger is set to engage the surrounding pipe wall and support the weight of the liner. Alternatively, the connection tubular is attached to the lower end of a previously installed casing or liner. Once the liner string is set, the liner may be cemented into place. The running tool is subsequently released from the liner and retrieved with the work string as it is withdrawn from the well bore.
Liner running tools conventionally include either hydraulic release means or mechanical release means. However, some liner running tools include both hydraulic and mechanical release means. Incorporating two different types of release means in a running tool is desirable given that trips into a well bore are expensive and time consuming. Thus, if the hydraulic release means fails, or if a liner must be reset, selective use of mechanical or hydraulic release means is desirable.
As described above, concentric liner strings are installed in the borehole as drilling progresses to increasing depths in a primary well bore or increasing lengths in a secondary well bore. Each new liner string must be run through the previously installed surface casing or liner string. Therefore, as successively smaller diameter liner strings are set, the flow area for the production of oil and gas is reduced. To maximize the production flow area, it is desirable to install a liner string with as large a diameter and length as possible so that the bottom of the formation can be reached with a comparatively larger diameter liner, thereby providing more flow area for the production of oil and gas.
However, traditional liner running tools have an outer diameter substantially the same size as the outer diameter of the liner string. Therefore, such running tools can only attach to the top of the liner string, and they act to “push” the liner string into the borehole. The longer the liner string, the more difficult it is for a traditional running tool to “push” the liner string into the borehole, especially in a lateral well bore. Therefore, it may be advantageous for a running tool to be releasably attachable to the liner string at any position along its length. Such a design would allow for the running tool to be connected near the lower end of the liner string, for example, so that a very long liner string may be “pulled” rather than “pushed” into an open borehole.
Further, some liner strings include features, such as slots or windows, which create structural weak points in the liner string. A running tool that could be attached to a liner string below a structural weak point, for example, would prevent stressing the weak point and buckling the liner string as it is being lowered into an open borehole. Therefore, a need exists for a liner running tool that may be releasably attached to a liner string at any location along its length.